This application claims the benefit of a priority under 35 USC 119 to French Patent Application No. 0103386 filed Mar. 13, 2001, the entire contents of which are hereby incorporated by reference.
The present invention relates to calibration techniques for image reconstruction of three-dimensional models from images obtained by X-ray tomography.
ConventionaUy, a tomography system comprises, as illustrated in FIG. 1, means for detection 1, (for example, a screen sensitive to X-rays), which may be fixed, and means for providing radiation 2, for example, a source of X-rays which can move and which adopts several positions with respect to the means for detection 1 and to a body or object that is to undergo radiography, the source 2 rotating about a fixed point 5 of the body or of the object.
Numerous techniques for reconstructing three-dimensional models from tomographic two-dimensional images are known. See, for example, a reconstruction algorithm, in xe2x80x9cImage reconstruction from projections: the fundamentals of computerized tomographyxe2x80x9dxe2x80x94T. Hermanxe2x80x94Academic Pressxe2x80x94Newxe2x80x94T. Hermanxe2x80x94Academic Press, New York (1980). Such reconstructions generally entail precise xe2x80x9cgeometric calibrationxe2x80x9d of the image-acquisition system, this calibration associating the three-dimensional space with the two-dimensional information supplied by the various two-dimensional projections. If this calibration is coarse, the quality of the three-dimensional model reconstructed will exhibit defects; in particular, small structures will appear fuzzy. In certain cases, this calibration is performed directly from information supplied by the image-acquisition system itself, such as the distance between the source and the means for detection, the angular positions of the source, etc. An alternative to this type of calibration comprises carrying out prior calibration of the image-acquisition geometry (without the patient) and in imposing precalibrated successive positions on the source. For mechanical reasons, these types of calibration do not give satisfactory precision or results.
Other calibration techniques use markers which act as references in space and which lie in the field of the X-rays during image acquisitions and therefore appear on the projected images. The positions of these markers in three-dimensional space are supposed to be known and the image acquisition geometry for each projection is deduced by inversion of a system of equations which is derived from the position of the markers on the projected images. In theory, techniques using these markers should give better precision than techniques not using markers. In practice, it is often difficult to determine precisely the position of the markers in space. In particular, during mammography, markers are fixed to a compression plate, whose position with respect to the detector is not known with great precision and which is likely to move slightly as the breast of the patient is compressed.
An embodiment of the invention is a method using markers to determine substantially precisely their position in space.
An embodiment of the invention is method of a calibration for reconstructing three-dimensional models from images obtained by a tomography apparatus comprising means for detection and means for providing radiation which can move with respect to the means for detection wherein a set of markers are placed in the field of the radiation and the positions of the projections of the markers onto the images acquired are processed so as to deduce from this the position of the means for providing radiation at the time of the acquisitions, the markers being carried on one and the same support. The markers are fixed with respect to the support and the positions of the projections of the markers onto the images acquired are processed so as to deduce from this the position of the markers in space.
In particular, in an embodiment of the invention, the positions of the projections of the markers onto the images acquired are processed so as to deduce from this conversion parameters which characterize a rigid displacement of the markers between their assumed initial position and a position adopted by the markers following a displacement.
In particular, according to an embodiment, the positions of the radiation source during various image acquisitions and the values of conversion parameters which, for all the markers and acquisitions, minimize an overall error which is a function of the distances between, on the one hand, the theoretical position of the projection of each marker into the image resulting from an acquisition and, on the other hand, its actual position in the image, are determined.
As understood, an embodiment of the method permits obtaining precise calibration substantially without the need initially to know the exact position of the markers in space.